libcurl-gnutls: add package, replaces gnURL

libcurl-gnutls: add package, replaces gnURL

The gnURL-fork of cURL is no longer maintained as cURL finally supports
probing and selecting the TLS implementation at run-time.

Hence just build a gnuTLS-backed variant of libcurl, use patchelf to
change the shared object name, call the result libcurl-gnutls and be
done. Other distributions have opted for similar solutions.

In future we could convert the curl package to provide build-variants
for each TLS implementation; however, this is out of the scope of the
needs of GNUnet which used to be only user of libgnurl.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>